Very low levels of n-butyl acrylate comonomer strongly affect residual stress relaxation in styrene/acrylic random copolymer films

Abstract

Processing-induced residual stresses in polymer films can cause microscopic defects and macroscopic dimensional changes. Although stress relaxation behavior of polymer films has been studied for several decades, little work has addressed random copolymers. We probed the residual stress relaxation in non-equilibrated random copolymer films of styrene (S) and n-butyl acrylate (nBA) using two optical techniques, ellipsometry and fluorescence. Both techniques show that incorporation of very low levels of nBA (2 mol% to 5 mol%) in S/nBA random copolymers can lead to dramatic increases in stress relaxation time of copolymer films relative to neat polystyrene films. For example, relative to polystyrene, the bulk-film stress relaxation time at 130 °C of 95/5 mol% S/nBA copolymer increases by factors of at least six as determined by ellipsometry and three as determined by fluorescence. Nanoconfined, 25-nm-thick films exhibit similar differences. This work provides implications for how low comonomer levels may deleteriously affect stress relaxation in polymer films, and we highlight that these findings are important for applications in which thin polymer films are required.